The present invention relates to a method of operating a piston expander.
To be able to realize a reduction in fuel, especially with mobile internal combustion engines, such as of vehicles, presently mainly two technical solutions are prioritized. In addition to the use of different hybrid concepts, which primarily present an opportunity for city and feeder traffic due to the braking and acceleration processes that occur in such situations, furthermore known are heat recovery systems that utilize the waste heat of an internal combustion engine in order to make additional drive energy available. Such systems for the utilization of waste heat present an opportunity for mobile internal combustion engines, especially for vehicles, that operate in long-distance traffic.
In such waste heat utilization systems, the waste heat available in the area of the internal combustion engine and/or of the exhaust gas discharge is at least partially transferred to a secondary heat circuit in which a heat-carrier fluid is circulated, and generally is at least partially vaporized in an evaporator, the vapor is expanded in an expansion unit, for example in a piston expander, and finally is again liquefied in a condenser. The mechanical work generated with the expansion unit is conveyed as additional work or energy to the drive system, in particular to a vehicle drive system. It is just as conceivable to utilize the mechanical work recovered by the waste heat utilization for driving other components, such as a fan or a compressor, or to generate electrical energy.
In this connection, a heat recovery system for an internal combustion engine is known from DE 10 2006 043 1.39 A1. With the aid of the described system, additional drive energy from the waste heat of the internal combustion engine and/or of the exhaust gas mechanism is made available to the vehicle. After expansion of the vaporous working medium in the expander, the working medium of the secondary heat circuit is conveyed into a condenser, in which the medium is liquefied accompanied by the release of heat, so that the corresponding steam circuit process is closed.
A piston of a lifting piston expansion engine is furthermore known from the currently not yet published European patent application 09009456.6, according to which an outer diameter of the piston neck is less than an outer diameter of the piston head and/or of the piston body, and at the same time the length of the piston neck corresponds approximately to the stroke of the piston. With the described technical solution, it is possible, with the aid of relatively simple structural means, to realize an effective utilization contained in the steam, and thus of the heat due to energy losses available in an internal combustion engine. The described construction of the piston ensures a gentle start of a piston expansion engine and an effective separation of the oil and steam circuits. The effective separation of the oil and steam circuits reliably prevents a reciprocal contamination of the circuits due to transfer of the respective medium.
The steam-piston expanders known in the state of the art generally operate in a two cycle process. In this connection, at the upper dead center position the live steam is introduced into a cylinder of the expander unit via an inlet valve, and in the following power stroke the steam is expanded accompanied by the release of energy. Finally, in the lower dead center position, the outlet valve is opened, and during the exhaust or expelling stroke the expanded steam is expelled out of the cylinder due to the movement of the piston from the lower dead center position to the upper dead center position. When the upper dead center position is reached, the outlet valve is closed and the corresponding cyclical process begins anew. As a function of the live steam parameters, as well as of the counter pressure on the outlet side, the compression rate must be designed such that the working medium is expanded in the power stroke to a suitable level.
If the compression rate is too low, the working medium, when the outlet valve is opened, has an overpressure relative to the counter pressure in the outlet line which, due to the potential possibility of reaching a greater expansion, has a negative effect upon the degree of efficiency of the cyclical process. If, on the other hand, the compression rate is too great, the working medium is expanded to a pressure below the counter pressure in the outlet line, which makes the expelling of the expanded steam more difficult and again has a negative impact upon the degree of efficiency of the cyclical process.
The use of steam-piston expanders during the utilization of waste heat from internal combustion engines requires a complex construction. In order to be able to fulfill all of the requirements with respect to weight, cost, durability and required service, generally stationary valves are used. As a consequence of this measure, there results in the upper dead center position a damage space that is relatively large and thus leads to low geometrical compression conditions. As a result, there is also the problem that not even the damage space can be adequately filled with high steam.
Proceeding from the state of the art and the problems that has been described, it is an object of the present invention to provide a method for the operation of a steam-piston expander unit that can be operated with a relatively high degree of efficiency. The method of the present application is intended in particular to reduce the quantity of live steam required for filling the piston without hereby significantly reducing the degree of efficiency of the cyclical process.